Thats a great price but its a rough job.
The down pipe (aka header pipe) is the greatest bear of all. The two bolts holding it to the exhaust manifold are likely rusted so badly they'll need to be ground, cut or acetylene torched off. You could try soaking the bolts with penetrating oil and attacking them with a breaker bar. If you round off the bolts, (which is likely) you'll need an exhaust shop. Trouble is, they likely won't stop at sending you off with a disconnected header pipe. Round the bolts, and youre committed to finishing the job!
Raising the Jeep securely on jackstands (centered on the underbody lift points) is a must for the install, but the removal part of the job could be done at ground level if you make an extra cut to break the taipipe in two sections. But getting the new tailpipe over the rear axle will require raising the rear end up. Make sure you know your lift points, not just any spot is ok so check your owner's or repair manual for your Jeep's lift points. Never use the jack alone to support the jeep though, leave that stunt to pit crews! Make sure you chock your front wheels first before raising the Jeep, and set your E-brake etc.
If you dont want to spend the money on a chain-type pipe cutter to section the header pipe in front of the cat, hacksawing squarely through an exhaust while under a jeep is possible. Ring the spot with tape where you're going to cut, wrapping around several times. The tape is a guide to make sure your cut is circular, and not twisted. It also protects the pipe from your first few attempts to start the saw's cut. If you saw through the aluminum skin of the exhaust pipe it will rust through there.
Once the new exhaust system is on its hangers, the cat slipped over the "downpipe" outlet, the muffler slipped over the cat outlet, and the tailpipe slipped over the muffler outlet, the clamps need to be positoned squarely and the clamps tightened to very slightly crush the pipes. Overtightening the clamps can cause a leak nearly as bad as undertightening.
These are only suggestions from my experience, your install may differ if you dont have a flange-type converter but already have a slip-on type. If you're replacing the down pipe, the new one may come with a flange, which you may need to cut off and flare to slip over the new cat's inlet.
I did look at the Ebay sytem, it is a slip-on and probably has a Walker muffler judging by the stamping on the case.